Chemical looping gasification of maceral from low-rank coal:Products distribution and kinetic analysis on vitrinite

Bo Zhang,Bolun Yang,Wei Guo,Song Wu,Jie Zhang,Zhiqiang Wu*

Shaanxi Key Laboratory of Energy Chemical Process Intensification,School of Chemical Engineering and Technology,Xi’an Jiaotong University,Xi’an,710049,China

Keywords:Coal Vitrinite Chemical looping process Gasification Products distribution Reaction kinetics

ABSTRACT The product distribution and kinetic analysis of low-rank coal vitrinite were investigated during the chemical looping gasification (CLG) process.The acid washing method was used to treat low-rank coal,and the density gradient centrifugation method was adopted to obtain the coal macerals.By combining thermogravimetric analysis and online mass spectrometry,the influence of the heating rate and oxygen carrier(Fe2O3)blending ratio on product distribution was discussed.The macroscopic kinetic parameters were solved by the Kissinger-Akahira-Sunose (KAS) method,and the main gaseous product formation kinetic parameters were solved by the iso-conversion method.The results of vitrinite during slow heating chemical looping gasification showed that the main weight loss interval was 400–600 °C,and the solid yield of sample vitrinite-Fe-10 at different heating rates was 64.30%–69.67%.When β=20 °C.min-1,the maximum decomposition rate of vitrinite-Fe-10 was -0.312%.min-1.The addition of Fe2O3 reduced the maximum decomposition rate,but by comparing the chemical looping conversion characteristic index,it could be inferred that the chemical looping gasification of vitrinite might produce volatile substances higher than the pyrolysis process of vitrinite alone.The average activation energy of the reaction was significantly reduced during chemical looping gasification of vitrinite,which was lower than the average activation energy of 448.69 kJ.mol-1 during the pyrolysis process of vitrinite alone.The gaseous products were mainly CO and CO2.When the heating rate was 10°C.min-1,the highest activation energy for CH4 formation was 21.353 kJ.mol-1,and the lowest activation energy for CO formation was 9.7333 kJ.mol-1.This study provides basic data for exploring coal chemical looping gasification mechanism and reactor design by studying the chemical looping gasification process of coal macerals.

1.Introduction

Although renewable energy sources are receiving more and more attention,coal is still a great alternative in the next decades among all of the fossil fuel sources.The coal source can meet about 23% of the total world primary energy demand [1].In 2015,the energy produced by coal combustion accounted for 28% of the world’s energy.Still,due to its high carbon content per unit of energy generation,its carbon dioxide emissions accounted for the total global quantity of 45% [2].The ‘13th Five-Year Plan for Coal Industry Development’points out that coal is still the primary energy source in the energy consumption structure of China[3].It is predicted that by 2050,the proportion of coal resources in China will always be as high as about 50% [4].Traditional utilization of coal has brought a series of environmental problems [5],which is also a significant source of greenhouse gases (GHGs) and air pollutants in China [6].How to achieve clean and efficient utilization of coal has become the primary research goal.The chemical looping process (CLP) is a novel thermochemical conversion technology that provides a new solution for the clean and efficient use of coal [7–9].Chemical looping gasification (CLG) can convert coal into syngas [10].CLG solves the problems of the high cost of oxygen production,the low comprehensive utilization rate of coal carbon,and difficult control of synthesis gas composition in traditional gasification technology [11].The oxygen carrier transports oxygen between the fuel reactor and the air reactor so that the target product CO and H2can be conveniently separated and stored without being doped with nitrogen and air [12,13].

Research on chemical looping gasification of coal has gradually increased in recent years [14–16].It is necessary to study the product distribution during coal chemical looping gasification.Compared with Yangchang Coal/Al2O3chemical looping gasification,An et al.[17] found the molar accumulation of H2decreased in the process of Yangchang Coal/CuFe2O4,while the molar accumulation of CO2,CO and CH4increased.Guo et al.[11]studied the CLG of Beisu bituminous coal and found that the carbon conversion efficiency was increased from 55.74% to 81% with increasing O/C ratio.In contrast,the content of H2first decreased and then increased.Hu et al.[18] found that the average carbon conversion rate and the cold gas efficiency of bituminous coal,lignite,and anthracite also increased with the temperature increased from 750°C to 950°C.All types of coal reacted with CaFe2O4to produce CO.Sub-bituminous coal,and lignite coal had better reactivity with CaFe2O4than the bituminous coal[19].Due to the diversity of coal,product distribution and reaction characteristics are different.Moreover,studies have shown that the coal pyrolysis process is greatly affected by the composition of coal macerals.Wang et al.[20] found that under low-temperature conditions,the inertinite of western China exhibited lower pyrolysis reactivity than the vitrinite.Li et al.[21] studied the effect of vacuum and pressurized conditions on the pyrolysis behavior of coal macerals,and the study showed that the tar yield under vacuum conditions was greatly improved.Strugnell et al.[22] found that in the process of rapid hydropyrolysis of raw coal and coal macerals,the effect of external hydrogen on the inert group was more prominent,which made the inert group more reactive and has a higher methane yield.However,little attention has been paid to the CLG of coal macerals.

Kinetic research is of great significance for analyzing reaction mechanisms and reactor design.At present,the kinetics of coal CLG is mainly based on the thermogravimetric analyzer (TGA) for macroscopic kinetics[23–25].Zhang[26]found that the fixed carbon conversion was the rate-determining step during the CLG process of coal with CuFe2O4as an oxygen carrier,and the corresponding activation energy was 188.48 kJ.mol-1.Lin et al.[27]used Zhundong’s coal ash as a novel oxygen carrier and found that the apparent activation energy of coal ash was about 26–29 kJ.mol-1via TGA,and the reduction process was divided into three stages.A series of kinetic models have been proposed based on the chemical looping combustion of coal [28–31],for example,the shrinking-core model[32,33].However,most of these kinetic models were used to solve the macroscopic kinetics,and the above kinetic analysis study obtained a single macroscopic kinetic parameter.Although many researchers have investigated the kinetic of coal CLG,little attention has been paid to the formation kinetics of gaseous products.Therefore,studying the kinetics of gaseous product formation has certain guiding significance for the design of CLG reactors.

The main purpose of this paper is to explore the distribution and kinetics of the products from the CLG of coal vitrinite,which is one of the coal macerals.Using thermogravimetric analyzer and online mass spectrometry study the product distribution under different heating rates (10 °C.min-1,20 °C.min-1,40°C.min-1) and different oxygen carrier blending ratios and the kinetic parameters of gaseous products were solved by the isoconversional method.This research is based on the main organic components of coal and has specific guiding significance for the selection of oxygen carriers and reactor design.

2.Materials and Methods

2.1.Materials

The raw material low-rank coal (LC) is collected from Shenfu coalfield,north of Shaanxi,China,which is used to prepare the coal macerals.The oxygen carrier is ferric oxide (Fe2O3).The coal sample is firstly ground by the sample preparation crusher 5E-PC1-100,and then the sample is sampled and sieved by the standard sieve shaker 5E-SS200 to obtain a 200 mesh sample,which is dried for later use.The ultimate and proximate analyses analysis of the LC is shown in Table 1.

Table1 Results of ultimate and proximate analyses on LC

2.2.Apparatus and methods

2.2.1.Preparation of acid washing coal and separation of coal macerals

The flow diagram of the preparation of acid washing coal and separation of coal macerals is shown in Fig.1.The specific steps are as follows:Sieving and selecting low-rank coal below 0.074 mm,taking 5 g coal samples and putting them in a polytetrafluoroethylene beaker,adding 60 ml of hydrochloric acid (HCl) with a concentration of 6 mol.L-1.After sealing,stirring at 60 °C for 4 h,and ultrasonic cleaning for 1 h.Washing with deionized water to neutrality,filter,and dry.Placing the obtained coal sample in a polytetrafluoroethylene beaker with 60 ml of 40% hydrofluoric acid(HF)solution and stirring at 60°C for 4 h,washing to neutral,filter,and dry.After stirring with 60 mL of 6 mol.L-1HCl for 3 h,washing to neutral,filter,and dry to obtain acid washing lowrank coal (ALC).Selecting a small amount of ALC in zinc chloride(ZnCl2) solution of different densities for the initial test,centrifuging for 10 min in a centrifuge at 7000 r.min-1,and measuring the quality of the suspended matter until the quality of the suspended matter no longer changes.Selecting the concentration at this time for the next step.According to the choice of the first step,mixing 100 ml of deionized water and 63.94 g of ZnCl2into a solution,the volume expands and stabilizes to room temperature,the density is 1.36 g.cm-3.Then add 1.2 g of ALC and centrifuging at 7000 r.min-1for 10 min.Filtering and washing the floating material and sinking material respectively to obtain the product vitrinite (abbreviated as JZ) 0.6125 g,inertinite (abbreviated as DZ) 0.2438 g.

Fig.1.The flow diagram of the preparation of acid washing coal and separation of coal macerals.

2.2.2.CLG of coal macerals

In the experiment,the thermogravimetric analyzer and online mass spectrometry are used for testing.The thermogravimetric analyzer is HCT-3 from Beijing Hengjiu Scientific Instrument Factory.The vitrinite and Fe2O3are mechanically mixed,and the mass fractions of Fe2O3are 10%(named JZ-Fe-10),30%(named JZ-Fe-30),and 50%(named JZ-Fe-50) respectively.Each experiment selects a sample of about 0.01 g.First purge with 60 ml.min-1of highpurity argon for 1 h to remove the air in the pipeline,and set it at different heating rates (10 °C.min-1,20 °C.min-1,40 °C.min-1)from 25 °C to 900 °C.The online mass spectrometer (AMETEK,American) was used to detect the gaseous products (H2,CO,CO2and CH4)produced by the reaction in time,and each was repeated three times [34].

2.2.3.Data processing and analysis methods

(1) Chemical looping conversion characteristic index [35,36]

where Rmaxis the maximum weight loss rate (mg.min-1),Tinis the initial reaction temperature (°C),Tmaxis the temperature to maximum weight loss rate(°C),ΔT1/2is the half-peak width when R/Rmax=0.5.

(2) Kinetics analysis

There were many thermogravimetric analyses based on carbonbased solid fuels,and the iso-conversion method is often used to calculate the kinetic characteristics parameters [35].The expression for solving dynamics using Kissinger-Akahira-Sunose (KAS)[37–39] is:

where β is the heating rate(°C.min-1),T is the reaction temperature(K),A is the frequency factor(s-1),R is the universal gas constant (8.314 kJ.mol-1.K-1),E is the activation energy (kJ.mol-1).A linear fit of lnand 1/RTαgives the value of E.

The combination of thermogravimetry and online mass spectrometry can solve the kinetic parameters of gaseous product formation.Kinetic method for gaseous product formation refers to the previous work [40].

The definition of α involved in solving the macroscopic activation energy and the activation energy of gaseous product formation are:

When solving the macroscopic activation energy,α refers to the percentage of reactant conversion under given reaction conditions.

where M0is the initial mass of reactant (kg),Mtis the mass of reactant at time t (kg),M∞is the mass of reactant at the end of the reaction (kg).

When solving the activation energy of gaseous product formation,conversion (α) is mentioned in Section 3.3.3.In this section,α refers to the percentage of a given gaseous product i.

where mtis the generation rate of product i at time t (kg),is the generation rate of product i at the end of the reaction at time t0.

3.Results and Discussion

3.1.Slow heating chemical looping conversion

Fig.2 shows the TG and DTG curves of JZ and Fe2O3at different heating rates.It can be seen from the TG curve that the sample starts to lose weight after 50 °C.The weight loss change was not apparent before 400 °C,and the weight loss was less than 10%.At this stage,the vitrinite mainly underwent slow pyrolysis [41],such as dehydration and softening.Gases such as CO2and CH4adsorbed in the pore structure of samples escaped with increasing temperature [42].The main weigh loss interval was 400–600 °C,and the bridge bond between the basic structural units of the coal was broken.Many violent chemical reactions occurred at this stage,accompanied by the release of a large amount of volatile matter and caused a massive weight loss.As the temperature continued to rise to 900°C,the weight loss continued to increase steadily,but the rate of weight loss gradually decreased because of the slow condensation process[42].The final solid yield of JZ-Fe-10,JZFe-30,JZ-Fe-50 at different heating rates was between 64.30%–69.67%,70.30%–73.22%,74.55%–77.58%.With the increase of Fe2O3blending ratio,the final solid yield increased,almost all exceed the final solid yield of JZ was in the range of 64.18% and 65.92%.Fig.2 also shows the DTG curve of JZ-Fe at different heating rates.The peak temperature of DTG gradually shifted to the hightemperature area with the increase of heating rate.And under the same mixing ratio,the increase of heating rate increased the maximum decomposition rate.

Table 2 shows the chemical looping conversion characteristic parameters at a heating rate of 20 °C.min-1.When β=20 °C.min-1,JZ-Fe-10 had a maximum decomposition rate of-0.312%.min-1.Under the same conditions,the maximum decomposition rate of JZ was-3.230%.min-1.It showed that the addition of Fe2O3reduced the maximum decomposition rate of JZ.However,by comparing the chemical looping conversion characteristic index,it can be inferred that the chemical looping conversion of JZ and Fe2O3to produce volatile substances may be higher than that of JZ pyrolysis alone.It can also be analyzed from Table 2,with the increase of the blending ratio,the maximum decomposition rate gradually decreased,the temperature corresponding to the maximum decomposition rate gradually increased,and the halfpeak width gradually increased.

Table2 JZ and Fe2O3 chemical looping conversion characteristic parameters β=20 °C.min-1

Fig.2.TG and DTG curves of JZ and Fe2O3 at different heating rates.

3.2.Kinetics of slow heating chemical looping conversion

Fig.3 shows the conversion of JZ and Fe2O3at different heating rates,and the changing trends are basically the same.The activation energy required for the reaction can be calculated by measuring the temperature at which the same conversion rate was reached under different heating rates.Using KAS method to solve the kinetic parameters of the slow heating chemical looping conversion process was shown in Table 3.When the Fe2O3blending ratio was 10%,30%,and 50% for the chemical looping conversion of JZ,the average activation energy was 194.51 kJ.mol-1,224.09 kJ.mol-1,and 359.40 kJ.mol-1,respectively.The averageactivation energy of chemical looping conversion was lower than the average activation energy of JZ reaction alone of 448.69 kJ.mol-1,indicating that chemical looping conversion can significantly reduce the activation energy of the reaction.Compared with the activation energy of the whole component of coal,it can be seen that the activation energy increased after adding Fe2O3,which may be due to the presence of metal oxides that can create diffusional resistance for the gasifying medium [24].

Table3 Kinetic parameters of the slow heating chemical looping conversion process

Fig.3.Conversion of JZ and Fe2O3 at different heating rates.

3.3.Gaseous product distribution and formation kinetics

3.3.1.Gaseous product distribution

Fig.4 shows the content and low heating value(LHV)of the four main gases obtained with different blending ratios.It can be seen from the figure that CO and CO2were the main gaseous products,and they accounted for more than 80% (vol) of the gaseous products.The functional groups dominated by aliphatic and aromatic carboxyl groups decomposed to release CO2[44].The source may be the primary pyrolysis reaction of JZ or the chemical looping gasification of pyrolysis products.In the sample gaseous product of JZ-Fe-10,with the increase of the heating rate,the CO content increased from 43.74% to 62.58%,the CO2content decreased from 39.42% to 21.35%,and the content of H2and CH4decreased firstly and then increased.CO2should have undergone a secondary reaction at a high heating rate,resulting in an increase in CO content.The LHV changed with the change of combustible gases (CO,CH4and H2) in the gaseous products.The LHV of the gaseous product gradually rised with the increase of the heating rate in Fig.4(a),from 9.817 MJ.m-3to 12.352 MJ.m-3.In Fig.4(b) and (c),when the heating rate was 20 °C.min-1,there is the lowest LHV of the gaseous product.Because the content of CO2was relatively high at this blending ratio,it had a certain inhibitory effect on the influence of the LHV.

3.3.2.Gaseous product release characteristics

The online mass spectrometry tracked the four main gaseous products in real time,and took the sample JZ-Fe-10 as an example.The results of the gas release characteristic curve were shown in Fig.5.Among the four gaseous products,CH4was produced first.According to the gas release characteristic curve,it can be seen from Fig.5 that the time required for various gas reactions decreased with the increase of the heating rate,that is,the high heating rate was beneficial to accelerate the reaction process and the generation of volatile matter.

3.3.3.Kinetic characteristics of gaseous product formation

Fig.6 shows the change in the conversion rate of CH4and CO overtime when the sample JZ-Fe-10 reacts at different heating rates.These two gaseous products had different reaction times,and the release of CH4was completed first.As the heating rate increased,various gaseous products during the sample reaction had a higher conversion rate within the same reaction time.This was because the temperature determined the reaction rate of the corresponding gas generated during the reaction and the concentration of the reaction precursor.The faster the rate,the shorter the time to reach the corresponding temperature.

According to the experimental data of thermogravimetry and online mass spectrometry,the results are shown in Fig.7.Based on the solution method of gaseous product kinetics,the relationship between conversion rate and the temperature was linearly fitted,and the activation energy and frequency factor of gaseous product formation were obtained as shown in Table 4.

Fig.4.Gaseous product distribution and low heating value of JZ and Fe2O3 at different heating rates:(a) JZ-Fe-10,(b) JZ-Fe-30,(c) JZ-Fe-50.

Fig.5.Four gaseous product release characteristics of JZ-Fe-10.(a) 10 °C.min-1,(b) 20 °C.min-1,(c) 40 °C.min-1.

Fig.6.The conversion rate of the sample JZ-Fe-10 with time,(a) CH4,(b) CO.

Table 4 shows the kinetic parameters of the four gaseous products of sample JZ-Fe-10 at different heating rates based on the thermogravimetric and online mass spectrometry.When the heating rate was 10°C.min-1,the highest activation energy for CH4formation was 21.353 kJ.mol-1,and the lowest activation energy for CO formation was 9.7333 kJ.mol-1.As the heating rate increases,the activation energy of CH4and CO2had a downward trend,and the activation energy of CO and H2had a rising trend.The comparison showed that CH4and CO had the lowest activation energy at 20 °C.min-1,which were 10.010 kJ.mol-1and 7.8422 kJ.mol-1,respectively.The minimum activation energy for the formation of CO2at a heating rate of 40 °C.min-1was 8.7880 kJ.mol-1,and the minimum activation energy for the formation of H2at a heating rate of 10°C.min-1was 10.602 kJ.mol-1.Studying the activation energy of gaseous products provided a basis for further research on the determination of experimental conditions.

Fig.7.The conversion rate of the sample JZ-Fe-10 four gases with temperature.(a) 10 °C.min-1,(b) 20 °C.min-1,(c) 40 °C.min-1.

Table4 Kinetic parameters of gaseous product formation

4.Conclusions

In this paper,the product distribution and kinetics of low-rank coal maceral (vitrinite) blended with Fe2O3were evaluated through a thermogravimetric analyzer combined with an online mass spectrometer.The research found that the main weight loss interval was 400–600 °C during slow heating chemical looping gasification coal vitrinite,and the solid yield of sample JZ-Fe-10 was 64.30%-69.67%.When the Fe2O3blending ratio was 10%,30%,and 50% of JZ,the average activation energy was 194.51 kJ.mol-1,224.09 kJ.mol-1,and 359.40 kJ.mol-1,respectively.The gaseous products were mainly CO and CO2,and the lowest activation energy for CO formation was 9.7333 kJ.mol-1.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledges the support of the National Natural Science Foundation of China (22038011,51976168),the K.C.Wong Education Foundation,China Postdoctoral Science Foundation (2019M653626),Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering(2020-KF-06),the Promotion Plan for Young People of Shaanxi Association for Science and Technology(20180402),and the Technology Foundation for Selected Overseas Chinese Scholar in Shaanxi Province (2018015).